This chapter discusses the propagation of electromagnetic waves as wave guide modes along axially uniform ducts. One of the most important applications of this branch of the electromagnetic theory is the propagation of very low-frequency waves, known as whistlers, in magnetospheric ducts. An important distinctive feature of the ducts is that they all are open guiding systems. Among the waves radiating outwards, there may be some waves which travel comparatively long distances along the duct prior to their eventual escape, or leakage, to the surrounding medium. The axis of a real duct is curved because of the curvature of the geomagnetic field line. A final point is that the inequalities, defining possible ranges of the propagation constants for the basic types of guided modes on sharply bounded uniform ducts with a step-like density profile, are applicable for nonuniform ducts with smooth monotonic variations of the density.